Why does String.prototype log it’s object like a standard object, while Array.prototype logs it’s object like a standard array?

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Because in a method call the this argument is always (in sloppy mode) casted to an object. What you see is a String object, which was produced from the "test" primitive string value. The array on which you call your method is already an object, so nothing happens and you just get the array as before.

If you use strict mode, this cast doesn’t happen:

String.prototype.test = function() {
    "use strict";
    console.log(this);
};
var str="test";
str.test(); // logs "test"

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